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EventLoop

ReactPHP's core reactor event loop that libraries can use for evented I/O.

Development version: This branch contains the code for the upcoming v3 release. For the code of the current stable v1 release, check out the 1.x branch.

The upcoming v3 release will be the way forward for this package. However, we will still actively support v1 for those not yet on the latest version. See also installation instructions for more details.

In order for async based libraries to be interoperable, they need to use the same event loop. This component provides a common LoopInterface that any library can target. This allows them to be used in the same loop, with one single run() call that is controlled by the user.

Table of contents

• Quickstart example
• Usage
  • Loop
    • Loop methods
    • Loop autorun
    • get()
  • Loop implementations
    • StreamSelectLoop
    • ExtEventLoop
    • ExtEvLoop
    • ExtUvLoop
  • LoopInterface
    • run()
    • stop()
    • addTimer()
    • addPeriodicTimer()
    • cancelTimer()
    • futureTick()
    • addSignal()
    • removeSignal()
    • addReadStream()
    • addWriteStream()
    • removeReadStream()
    • removeWriteStream()
• Install
• Tests
• License
• More

Quickstart example

Here is an async HTTP server built with just the event loop.

See also the examples.

Usage

Typical applications would use the Loop class to use the default event loop like this:

As an alternative, you can also explicitly create an event loop instance at the beginning, reuse it throughout your program and finally run it at the end of the program like this:

While the former is more concise, the latter is more explicit. In both cases, the program would perform the exact same steps.

1. The event loop instance is created at the beginning of the program. This is implicitly done the first time you call the Loop class (or by manually instantiating any of the loop implementations).
2. The event loop is used directly or passed as an instance to library and application code. In this example, a periodic timer is registered with the event loop which simply outputs Tick every fraction of a second until another timer stops the periodic timer after a second.
3. The event loop is run at the end of the program. This is automatically done when using the run() call at the end of the program.

As of v1.2.0, we highly recommend using the Loop class. The explicit loop instructions are still valid and may still be useful in some applications, especially for a transition period towards the more concise style.

Loop

The Loop class exists as a convenient global accessor for the event loop.

Loop methods

The Loop class provides all methods that exist on the LoopInterface as static methods:

• run()
• stop()
• addTimer()
• addPeriodicTimer()
• cancelTimer()
• futureTick()
• addSignal()
• removeSignal()
• addReadStream()
• addWriteStream()
• removeReadStream()
• removeWriteStream()

If you're working with the event loop in your application code, it's often easiest to directly interface with the static methods defined on the Loop class like this:

On the other hand, if you're familiar with object-oriented programming (OOP) and dependency injection (DI), you may want to inject an event loop instance and invoke instance methods on the LoopInterface like this:

Each static method call will be forwarded as-is to the underlying event loop instance by using the Loop::get() call internally. See LoopInterface for more details about available methods.

Loop autorun

When using the Loop class, it will automatically execute the loop at the end of the program. This means the following example will schedule a timer and will automatically execute the program until the timer event fires:

As of v1.2.0, we highly recommend using the Loop class this way and omitting any explicit run() method is still valid and may still be useful in some applications, especially for a transition period towards the more concise style.

If you don't want the Loop to run automatically, you can either explicitly stop() it. This can be useful if you're using a global exception handler like this:

get()

The get(): LoopInterface method can be used to get the currently active event loop instance.

This method will always return the same event loop instance throughout the lifetime of your application.

This is particularly useful if you're using object-oriented programming (OOP) and dependency injection (DI). In this case, you may want to inject an event loop instance and invoke instance methods on the LoopInterface like this:

See LoopInterface for more details about available methods.

Loop implementations

In addition to the LoopInterface, there are a number of event loop implementations provided.

All of the event loops support these features:

• File descriptor polling
• One-off timers
• Periodic timers
• Deferred execution on future loop tick

For most consumers of this package, the underlying event loop implementation is an implementation detail. You should use the Loop class to automatically create a new instance.

Advanced! If you explicitly need a certain event loop implementation, you can manually instantiate one of the following classes. Note that you may have to install the required PHP extensions for the respective event loop implementation first or they will throw a BadMethodCallException on creation.

StreamSelectLoop

A stream_select() based event loop.

This uses the stream_select() function and is the only implementation that works out of the box with PHP.

This event loop works out of the box on any PHP version. This means that no installation is required and this library works on all platforms and supported PHP versions. Accordingly, the Loop class will use this event loop by default if you do not install any of the event loop extensions listed below.

Under the hood, it does a simple select system call. This system call is limited to the maximum file descriptor number of FD_SETSIZE (platform dependent, commonly 1024) and scales with O(m) (m being the maximum file descriptor number passed). This means that you may run into issues when handling thousands of streams concurrently and you may want to look into using one of the alternative event loop implementations listed below in this case. If your use case is among the many common use cases that involve handling only dozens or a few hundred streams at once, then this event loop implementation performs really well.

If you want to use signal handling (see also addSignal() below), this event loop implementation requires ext-pcntl. This extension is only available for Unix-like platforms and does not support Windows. It is commonly installed as part of many PHP distributions. If this extension is missing (or you're running on Windows), signal handling is not supported and throws a BadMethodCallException instead.

This event loop is known to rely on wall-clock time to schedule future timers when using any version before PHP 7.3, because a monotonic time source is only available as of PHP 7.3 (hrtime()). While this does not affect many common use cases, this is an important distinction for programs that rely on a high time precision or on systems that are subject to discontinuous time adjustments (time jumps). This means that if you schedule a timer to trigger in 30s on PHP < 7.3 and then adjust your system time forward by 20s, the timer may trigger in 10s. See also addTimer() for more details.

ExtEventLoop

An ext-event based event loop.

This uses the event PECL extension, that provides an interface to libevent library. libevent itself supports a number of system-specific backends (epoll, kqueue).

This loop is known to work with PHP 7.1 through PHP 8+.

ExtEvLoop

An ext-ev based event loop.

This loop uses the ev PECL extension, that provides an interface to libev library. libev itself supports a number of system-specific backends (epoll, kqueue).

This loop is known to work with PHP 7.1 through PHP 8+.

ExtUvLoop

An ext-uv based event loop.

This loop uses the uv PECL extension, that provides an interface to libuv library. libuv itself supports a number of system-specific backends (epoll, kqueue).

This loop is known to work with PHP 7.1 through PHP 8+.

LoopInterface

run()

The run(): void method can be used to run the event loop until there are no more tasks to perform.

For many applications, this method is the only directly visible invocation on the event loop. As a rule of thumb, it is usually recommended to attach everything to the same loop instance and then run the loop once at the bottom end of the application.

This method will keep the loop running until there are no more tasks to perform. In other words: This method will block until the last timer, stream and/or signal has been removed.

Likewise, it is imperative to ensure the application actually invokes this method once. Adding listeners to the loop and missing to actually run it will result in the application exiting without actually waiting for any of the attached listeners.

This method MUST NOT be called while the loop is already running. This method MAY be called more than once after it has explicitly been stop()ped or after it automatically stopped because it previously did no longer have anything to do.

stop()

The stop(): void method can be used to instruct a running event loop to stop.

This method is considered advanced usage and should be used with care. As a rule of thumb, it is usually recommended to let the loop stop only automatically when it no longer has anything to do.

This method can be used to explicitly instruct the event loop to stop:

Calling this method on a loop instance that is not currently running or on a loop instance that has already been stopped has no effect.

addTimer()

The addTimer(float $interval, callable $callback): TimerInterface method can be used to enqueue a callback to be invoked once after the given interval.

The second parameter MUST be a timer callback function that accepts the timer instance as its only parameter. If you don't use the timer instance inside your timer callback function you MAY use a function which has no parameters at all.

The timer callback function MUST NOT throw an Exception. The return value of the timer callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

This method returns a timer instance. The same timer instance will also be passed into the timer callback function as described above. You can invoke cancelTimer to cancel a pending timer. Unlike addPeriodicTimer(), this method will ensure the callback will be invoked only once after the given interval.

See also example #1.

If you want to access any variables within your callback function, you can bind arbitrary data to a callback closure like this:

This interface does not enforce any particular timer resolution, so special care may have to be taken if you rely on very high precision with millisecond accuracy or below. Event loop implementations SHOULD work on a best effort basis and SHOULD provide at least millisecond accuracy unless otherwise noted. Many existing event loop implementations are known to provide microsecond accuracy, but it's generally not recommended to rely on this high precision.

Similarly, the execution order of timers scheduled to execute at the same time (within its possible accuracy) is not guaranteed.

This interface suggests that event loop implementations SHOULD use a monotonic time source if available. Given that a monotonic time source is only available as of PHP 7.3 by default, event loop implementations MAY fall back to using wall-clock time. While this does not affect many common use cases, this is an important distinction for programs that rely on a high time precision or on systems that are subject to discontinuous time adjustments (time jumps). This means that if you schedule a timer to trigger in 30s and then adjust your system time forward by 20s, the timer SHOULD still trigger in 30s. See also event loop implementations for more details.

addPeriodicTimer()

The addPeriodicTimer(float $interval, callable $callback): TimerInterface method can be used to enqueue a callback to be invoked repeatedly after the given interval.

The second parameter MUST be a timer callback function that accepts the timer instance as its only parameter. If you don't use the timer instance inside your timer callback function you MAY use a function which has no parameters at all.

The timer callback function MUST NOT throw an Exception. The return value of the timer callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

This method returns a timer instance. The same timer instance will also be passed into the timer callback function as described above. Unlike addTimer(), this method will ensure the callback will be invoked infinitely after the given interval or until you invoke cancelTimer.

See also example #2.

If you want to limit the number of executions, you can bind arbitrary data to a callback closure like this:

This interface does not enforce any particular timer resolution, so special care may have to be taken if you rely on very high precision with millisecond accuracy or below. Event loop implementations SHOULD work on a best effort basis and SHOULD provide at least millisecond accuracy unless otherwise noted. Many existing event loop implementations are known to provide microsecond accuracy, but it's generally not recommended to rely on this high precision.

Similarly, the execution order of timers scheduled to execute at the same time (within its possible accuracy) is not guaranteed.

This interface suggests that event loop implementations SHOULD use a monotonic time source if available. Given that a monotonic time source is only available as of PHP 7.3 by default, event loop implementations MAY fall back to using wall-clock time. While this does not affect many common use cases, this is an important distinction for programs that rely on a high time precision or on systems that are subject to discontinuous time adjustments (time jumps). This means that if you schedule a timer to trigger in 30s and then adjust your system time forward by 20s, the timer SHOULD still trigger in 30s. See also event loop implementations for more details.

Additionally, periodic timers may be subject to timer drift due to re-scheduling after each invocation. As such, it's generally not recommended to rely on this for high precision intervals with millisecond accuracy or below.

cancelTimer()

The cancelTimer(TimerInterface $timer): void method can be used to cancel a pending timer.

See also example #2.

Calling this method on a timer instance that has not been added to this loop instance or on a timer that has already been cancelled has no effect.

futureTick()

The futureTick(callable $listener): void method can be used to schedule a callback to be invoked on a future tick of the event loop.

This works very much similar to timers with an interval of zero seconds, but does not require the overhead of scheduling a timer queue.

The tick callback function MUST be able to accept zero parameters.

The tick callback function MUST NOT throw an Exception. The return value of the tick callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

If you want to access any variables within your callback function, you can bind arbitrary data to a callback closure like this:

Unlike timers, tick callbacks are guaranteed to be executed in the order they are enqueued. Also, once a callback is enqueued, there's no way to cancel this operation.

This is often used to break down bigger tasks into smaller steps (a form of cooperative multitasking).

See also example #3.

addSignal()

The addSignal(int $signal, callable $listener): void method can be used to register a listener to be notified when a signal has been caught by this process.

This is useful to catch user interrupt signals or shutdown signals from tools like supervisor or systemd.

The second parameter MUST be a listener callback function that accepts the signal as its only parameter. If you don't use the signal inside your listener callback function you MAY use a function which has no parameters at all.

The listener callback function MUST NOT throw an Exception. The return value of the listener callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

See also example #4.

Signaling is only available on Unix-like platforms, Windows isn't supported due to operating system limitations. This method may throw a BadMethodCallException if signals aren't supported on this platform, for example when required extensions are missing.

Note: A listener can only be added once to the same signal, any attempts to add it more than once will be ignored.

removeSignal()

The removeSignal(int $signal, callable $listener): void method can be used to remove a previously added signal listener.

Any attempts to remove listeners that aren't registered will be ignored.

addReadStream()

Advanced! Note that this low-level API is considered advanced usage. Most use cases should probably use the higher-level readable Stream API instead.

The addReadStream(resource $stream, callable $callback): void method can be used to register a listener to be notified when a stream is ready to read.

The first parameter MUST be a valid stream resource that supports checking whether it is ready to read by this loop implementation. A single stream resource MUST NOT be added more than once. Instead, either call removeReadStream() first or react to this event with a single listener and then dispatch from this listener. This method MAY throw an Exception if the given resource type is not supported by this loop implementation.

The second parameter MUST be a listener callback function that accepts the stream resource as its only parameter. If you don't use the stream resource inside your listener callback function you MAY use a function which has no parameters at all.

The listener callback function MUST NOT throw an Exception. The return value of the listener callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

If you want to access any variables within your callback function, you can bind arbitrary data to a callback closure like this:

See also example #11.

You can invoke removeReadStream() to remove the read event listener for this stream.

The execution order of listeners when multiple streams become ready at the same time is not guaranteed.

Some event loop implementations are known to only trigger the listener if the stream becomes readable (edge-triggered) and may not trigger if the stream has already been readable from the beginning. This also implies that a stream may not be recognized as readable when data is still left in PHP's internal stream buffers. As such, it's recommended to use stream_set_read_buffer($stream, 0); to disable PHP's internal read buffer in this case.

addWriteStream()

Advanced! Note that this low-level API is considered advanced usage. Most use cases should probably use the higher-level writable Stream API instead.

The addWriteStream(resource $stream, callable $callback): void method can be used to register a listener to be notified when a stream is ready to write.

The first parameter MUST be a valid stream resource that supports checking whether it is ready to write by this loop implementation. A single stream resource MUST NOT be added more than once. Instead, either call removeWriteStream() first or react to this event with a single listener and then dispatch from this listener. This method MAY throw an Exception if the given resource type is not supported by this loop implementation.

The second parameter MUST be a listener callback function that accepts the stream resource as its only parameter. If you don't use the stream resource inside your listener callback function you MAY use a function which has no parameters at all.

The listener callback function MUST NOT throw an Exception. The return value of the listener callback function will be ignored and has no effect, so for performance reasons you're recommended to not return any excessive data structures.

If you want to access any variables within your callback function, you can bind arbitrary data to a callback closure like this:

See also example #12.

You can invoke removeWriteStream() to remove the write event listener for this stream.

The execution order of listeners when multiple streams become ready at the same time is not guaranteed.

removeReadStream()

The removeReadStream(resource $stream): void method can be used to remove the read event listener for the given stream.

Removing a stream from the loop that has already been removed or trying to remove a stream that was never added or is invalid has no effect.

removeWriteStream()

The removeWriteStream(resource $stream): void method can be used to remove the write event listener for the given stream.

Removing a stream from the loop that has already been removed or trying to remove a stream that was never added or is invalid has no effect.

Install

The recommended way to install this library is through Composer. New to Composer?

Once released, this project will follow SemVer. At the moment, this will install the latest development version:

See also the CHANGELOG for details about version upgrades.

This project aims to run on any platform and thus does not require any PHP extensions and supports running on PHP 7.1 through current PHP 8+. It's highly recommended to use the latest supported PHP version for this project.

Installing any of the event loop extensions is suggested, but entirely optional. See also event loop implementations for more details.

Tests

To run the test suite, you first need to clone this repo and then install all dependencies through Composer:

To run the test suite, go to the project root and run:

License

MIT, see LICENSE file.

More

• See our Stream component for more information on how streams are used in real-world applications.
• See our users wiki and the dependents on Packagist for a list of packages that use the EventLoop in real-world applications.